Wendong Wang

From the thermodynamic point of view, most materials are either in their equilibrium states, such as inorganic and organic crystals, or in kinetically trapped non-equilibrium states, such as porous materials. Life, on the other hand, is in dissipative non-equilibrium (or dynamic) state, with hierarchically ordered complex structures and highly coordinated functions. Developing dynamic materials systems where structural orders are sustained by continuous energy input and dissipation will not only introduce a new paradigm in materials science but also impact robotics. In the context of robotics, a micro-robot collective is a dynamic and programmable materials system, where fundamental physical and chemical principles guide the designs of local interactions and global behaviors. Through judicious selections of physiochemical forces from the nanometer scale to the millimeter scale, we are developing a platform of micro-robot collectives that not only advances our understanding of collective systems in general but will also help address critical challenges facing society, such as healthcare and clean water, in the future.

collective systems dynamic materials programmable self-assembly

Exploring collective behaviors using the platform of dynamic and programmable self-assembly of spinning micro-rafts at the air-water interface.

My current research interests lie at the intersection of materials synthesis/fabrication and non-equilibrium systems, including additive manufacturing and high throughput materials discovery through machine learning, morphogenesis and programmable self-assembly, nanofluidics and clean water, micro-robotics and dynamic materials systems.

Developing adaptive materials with geometries that change in response to external stimuli provides fundamental insights into the links between the physical forces involved and the resultant morphologies and creates a foundation for technologically relevant dynamic systems1,2. In particular, reconfigurable surface topography as a means to control interfacial properties 3 has recently been explored using responsive gels 4 , shape-memory polymers 5 , liquid crystals6-8 and hybrid composites9-14, including magnetically active slippery surfaces12-14. However, these designs exhibit a limited range of topographical changes and thus a restricted scope of function. Here we introduce a hierarchical magneto-responsive composite surface, made by infiltrating a ferrofluid into a microstructured matrix (termed ferrofluid-containing liquid-infused porous surfaces, or FLIPS). We demonstrate various topographical reconfigurations at multiple length scales and a broad range of associated emergent behaviours. An applied magnetic-field gradient induces the movement of magnetic nanoparticles suspended in the ferrofluid, which leads to microscale flow of the ferrofluid first above and then within the microstructured surface. This redistribution changes the initially smooth surface of the ferrofluid (which is immobilized by the porous matrix through capillary forces) into various multiscale hierarchical topographies shaped by the size, arrangement and orientation of the confining microstructures in the magnetic field. We analyse the spatial and temporal dynamics of these reconfigurations theoretically and experimentally as a function of the balance between capillary and magnetic pressures15-19 and of the geometric anisotropy of the FLIPS system. Several interesting functions at three different length scales are demonstrated: self-assembly of colloidal particles at the micrometre scale; regulated flow of liquid droplets at the millimetre scale; and switchable adhesion and friction, liquid pumping and removal of biofilms at the centimetre scale. We envision that FLIPS could be used as part of integrated control systems for the manipulation and transport of matter, thermal management, microfluidics and fouling-release materials.

2017

Science Advances, 3(5):e1602522, American Association for the Advancement of Science, May 2017 (article)

Abstract

Dynamic self-assembled material systems constantly consume energy to maintain their spatiotemporal structures and functions. Programmable self-assembly translates information from individual parts to the collective whole. Combining dynamic and programmable self-assembly in a single platform opens up the possibilities to investigate both types of self-assembly simultaneously and to explore their synergy. This task is challenging because of the difficulty in finding suitable interactions that are both dissipative and programmable. We present a dynamic and programmable self-assembling material system consisting of spinning at the air-water interface circular magnetic micro-rafts of radius 50 μm and with cosinusoidal edge-height profiles. The cosinusoidal edge-height profiles not only create a net dissipative capillary repulsion that is sustained by continuous torque input but also enable directional assembly of micro-rafts. We uncover the layered arrangement of micro-rafts in the patterns formed by dynamic self-assembly and offer mechanistic insights through a physical model and geometric analysis. Furthermore, we demonstrate programmable self-assembly and show that a 4-fold rotational symmetry encoded in individual micro-rafts translates into 90° bending angles and square-based tiling in the assembled structures of micro-rafts. We anticipate that our dynamic and programmable material system will serve as a model system for studying nonequilibrium dynamics and statistical mechanics in the future

Our goal is to understand the principles of Perception, Action and Learning in autonomous systems that successfully interact with complex environments and to use this understanding to design future systems